Attached patch adds to the index page a list of 5 steps necessary to set up Python and make a patch.  These informations are currently scattered around a few pages and many contributors are already familiar with Mercurial and the process of creating patches, so they just need to know the address where to clone and the commands to compile Python and run the tests.
The list also includes links to the other sections, so users can find more detailed information if they need them (this is also valid for the second point: the "configure" command is specific to Unix, but the link has information about Windows too).
All this should make the devguide a bit more usable.

(A bit of background: during the sprints at PyCon FI I wrote the steps 1-3 on the whiteboard, and they worked well for everyone.  Most of them managed to get something done with no further assistance, even if a few people were not so familiar with Mercurial.)